This application claims benefit of priority under 35USC xc2xa7119 to Japanese patent application No. 2001-291223, filed on Sep. 25, 2001, the contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a charged particle beam apparatus, a pattern measuring method and a pattern drawing method.
2. Description of the Prior Art
A charged particle beam apparatus is widely used in a semiconductor fabricating process as a scanning electron microscope which observes a pattern formed on a semiconductor wafer and an electron beam lithography apparatus which draws a pattern on a semiconductor wafer. In using such a charged particle beam apparatus, prior to observation and drawing, the focal position of a charged particle beam may be fine-adjusted to decide a measuring position and a patterning position. In a prior art, a height detector of an optical type or an electrostatic capacity type is used to detect a height of a sample surface, and a calibration parameter of the detected sample height and the focal position of a charged particle beam is calculated to set a focus control current or a focus control voltage so that the charged particle beam is focused on the sample surface, thereby coarse-adjusting the focal position.
As an example of a prior art apparatus and method which coarse-adjust a focal position, there is Japanese patent application Laid-Open No. 1999-149895. Japanese patent application Laid-Open No. 1999-149895 discloses a height detector which detects the position of a lattice-like light flux reflected from the surface of a sample by projecting the lattice-like light flux onto the sample from above the sample in a slanting direction to measure the height of the surface of the sample from a change of the position; focus control means which converges an electron beam on the surface of the sample based on the measured height of the surface of the sample; and deflection control means which calibrates image distortion including a magnification error of an electron beam image caused based on the focus control.
A charged particle beam apparatus typically has a sample stage which supports and/or fixes a sample. To stabilize the incident voltage of a charged particle beam onto the sample, a ground or a constant voltage is applied to the sample stage. This holds the potential of the sample constant.
However, there is a case in which a suitable voltage cannot be applied to the sample since the periphery of the sample is covered by an insulator film or a case in which the irradiated region can be electrically charged by irradiation of the charged particle beam so that the potential of the sample can be varied locally. In such cases, magnification variation becomes a problem. This point will be described by taking an electron beam apparatus as an example of a charged particle beam apparatus with reference to FIGS. 6 to 7B. In the following drawings, like parts are indicated by the same reference numerals and repetitive descriptions thereof are suitably omitted.
FIG. 6 is a block diagram for explaining problems in the prior art electron beam apparatus. The drawing shows that deviation between a sample surface position obtained by a sample height detector and an electron beam focal position results in a magnification error. Electron beam EB produced from an electron beam generation source 10 is converged by a condenser lens 12, which is then deflected by two deflectors 14a, 14b to be focused so as to focus on the surface of a sample 20 by an objective lens 16. The potential of the sample 20 is typically constant. The electron beam EB is focused on the surface of the sample. The sample surface position obtained from the height detector of an optical type or an electrostatic capacity type is thus matched with the electron beam focal position (The sample position is referred to as Z0). When the sample potential is varied, however, the incident voltage onto a sample 20 is changed due to the influence. There occurs deviation between the sample surface position Z0, and the electron beam focal position (The electron beam focal position is referred to as Z1). Specifically, along with the variation in the sample potential, the sample position is changed imaginarily. The focus control current of the objective lens 16 is varied from the original current value I0 to I1 corresponding to the imaginary position. In accordance with this, the deflection width of the electron beam EB is varied from the original width W0 to width W1. Image distortion including a magnification error is thus caused as long as the magnification compensation of an electron beam image is not properly performed, even if the focal position of the electron beam EB can be detected in this state. FIGS. 7A and 7B show distortion of the electron beam image caused by the deviation of the sample position. When the electron beam image is used to measure the size of pattern P, the case in which a voltage is applied properly to the sample 20 (Ra) and the case in which a voltage is not applied properly to the sample 20 (Rb) are largely different in the size of the irradiated region of the electron beam EB, as shown in FIG. 7A. As a result the magnification of the electron beam image obtained from a secondary electron detector 62 and a SEM image signal processing part 64 (see FIG. 6) is varied. That is, as is apparent from the contrast of image Im (Z0) with image Im (Z1) shown in FIG. 7B, the size of the pattern P displayed on the screen is varied. As a result, the measured value is changed so that sufficient measurement reproducibility cannot be obtained.
To properly apply a voltage to the sample 20, there is a method in which removes the insulator film of the periphery of the sample 20. However, the sample can be scratched and dust can be produced. In consideration of these, this method is lacking in practicality.
According to a first aspect of the present invention, there is provided a charged particle beam apparatus comprising: a charged particle source which generates a charged particle beam; a condenser lens which converges the charged particle beam; a deflector which deflects the charged particle beam to scan a sample with the charged particle beam; an objective lens which converges the charged particle beam on the surface of the sample; a sample position imaginary variation detection part which detects an imaginary variation of a sample position in height caused by variation of the focal position of the charged particle beam due to variation in the potential of the sample; and a sample position imaginary variation compensation part which compensates for the detected imaginary variation of the sample position.
According to a second aspect of the present invention, there is provided a pattern measuring method using a charged particle beam apparatus, the charged particle beam apparatus comprising, a charged particle source which generates a charged particle beam, a condenser lens which converges the charged particle beam, deflector which deflects the charged particle beam for scanning a sample with the charged particle beam, an objective lens which converges the charged particle beam on the surface of the sample, and a measuring part which measures a pattern formed on the surface of the sample on the basis of a secondary charged particle or a reflective charged particle which are produced from the surface of the sample by irradiation of the charged particle beam, the method comprises: detecting an imaginary variation of a sample position caused by variation of the focal position of the charged particle beam due to variation in the potential of the sample; and compensating for the imaginary variation of the sample position.
According to a third aspect of the present invention, there is provided a method for writing a pattern on a surface of a sample using a charged particle beam apparatus, the charged particle beam apparatus comprising a charged particle source which generates a charged particle beam, a condenser lens which converges the charged particle beam, a deflector which deflects the charged particle beam to scan the sample with the charged particle beam, and an objective lens which converges the charged particle beam on the surface of the sample, the method comprises: adjusting a focal position of the charged particle beam by adjusting a current or a voltage given to the objective lens to search the focal position of the charged particle beam and obtaining a focus control current value or a focus control voltage value for the objective lens when the charged particle beam is focused on the surface of the sample, as a first focus control current value or a first focus control voltage value, respectively; detecting the height of the surface of the sample; calculating a focus control current value or a focus control voltage value given to the objective lens when a focal position of the charged particle beam corresponds to the detected height of the surface of the sample, as a second current value or a second voltage value, respectively; calculating a focus control current difference which is a difference between the first focus control current value and the second focus control current value or a focus control voltage difference which is a difference between the first focus control voltage value and the second focus control voltage value; calculating a magnification variation of the charged particle beam on the basis of the focus control current difference or the focus control voltage differential; and generating a deflection control signal as a compensation signal which compensates for a control signal to the deflectors corresponding to the calculated magnification variation; and writing a pattern on the sample with the charged particle beam while compensating for the deflection amount of the deflectors on the basis of the deflection control signal.